IMS online log data set (OLDS)
IMS writes log records to a DASD data set called the online log data set (OLDS).
The OLDS is made up of multiple data sets written in wraparound form. Using more than one OLDS enables IMS to continue logging when the first OLDS is full. Also, if an I/O error occurs while writing to an OLDS, IMS can continue logging by isolating the OLDS where the problem occurred and switching to another one.
IMS can write committed log records to the write-ahead data set (WADS) so that these records are externalized to avoid the need to write partially filled and padded log blocks to the OLDS. The WADS is described in IMS write-ahead data set (WADS).
When the OLDS is full, it is archived to the system log data set (SLDS). How frequently the OLDS is archived depends on whether you specified automatic archiving using the ARC=parameter in the DBC JCL. You can specify ARC=1 through ARC=99. Automatic archiving takes place only when the number of OLDS you specified is full. The system reuses the OLDS after it has been archived. An SLDS can be on DASD or on tape. The contents are used as input to the database recovery process.
IMS archives the OLDS using the log archive utility (DFSUARC0). During archiving, IMS can write a subset of the log records it writes to the SLDS to the recovery log data set (RLDS). This subset consists only of the log records required to perform a database recovery.
During logging, IMS writes system checkpoint ID information (including OLDS positioning information) to the restart data set (RDS). IMS uses the RDS during the restart process to determine from which checkpoint to begin a restart. (See IMS: Operations and automation for further guidance about the RDS.)